1. Field of the Invention
The present invention relates to a plasma processing method and a plasma processing apparatus, and particularly to a plasma processing method and a plasma processing apparatus related to plasma etching.
2. Description of the Related Art
One of plasma etching apparatuses currently used for the mass production of semiconductor devices is an electron cyclotron resonance (hereinafter referred to as ECR) type apparatus. In this plasma etching apparatus, by applying a magnetic field to a plasma, and setting magnetic field strength so that the frequency of a microwave resonates with the cyclotron frequency of electrons, a high density plasma can be generated.
With the miniaturization of semiconductor devices in recent years, the thickness of the gate oxide film has been 2 nm or less. Therefore, it has been necessary to achieve the controllability of plasma etching processing, and a high selection ratio of the gate oxide film to the silicon film.
One of techniques for achieving the high precision plasma etching is a plasma etching method using pulsed discharge. For example, Japanese Patent Laid-Open Publication No. 09-185999 (Patent Document 1) discloses a method for attaining high precision etching by pulse-modulating a plasma, while measuring radical density in the plasma, to control the radical density.
In addition, Japanese Patent Laid-Open Publication No. 09-092645 (Patent Document 2) discloses a method for pulse-modulating a plasma and simultaneously synchronizing the phase of a radio-frequency bias applied to a wafer with the ON-OFF of the plasma to control the temperature of electrons in the plasma to prevent the dielectric breakdown of an oxide film on the wafer being processed.
In addition, Japanese Patent Laid-Open Publication No. 08-181125 (Patent Document 3) discloses a method for preventing the dielectric breakdown of an oxide film and simultaneously attaining high rate anisotropic etching by pulse-modulating a plasma at 10 to 100 μs and applying a radio-frequency bias at 600 KHz or less to a wafer.
In addition, Japanese Patent Laid-Open Publication No. 06-267900 (Patent Document 4) discloses a method for pulse-modulating a microwave for generating a plasma, to control radicals, and suppressing the instability of the plasma to decrease ion temperature.
Generally, an ECR type plasma etching apparatus has three problems as shown below.
The first problem is that there is a case where a lower density region (lower microwave power) is required according to processing, such as for an improvement in perpendicularity, but when the microwave power is decreased in order to decrease the plasma density, the production of a plasma is difficult.
The second problem is that when a discharge test is performed changing the power of the microwave, there is an unstable region in which the light emission of the plasma is seen as flickering in visual inspection or measurement by a photodiode or the like, depending on the microwave power. There is no reproducibility of characteristics, such as an etching rate, in this region, and therefore, etching conditions are set avoiding the unstable region, that is, a narrow process window is set in process development.
The discharge flicker of interest in the present invention is a phenomenon in which the electric field strength distribution within the chamber changes, depending on the microwave power, abnormal discharge is generated, for example, near the sample stage or near the microwave transmission window, in connection with the chamber shape, and blinking can be observed by visual inspection.
The third problem is that there is a high selection ratio region on the high microwave power side (high density region), but on the high microwave power side, a cut-off phenomenon occurs with an increase in plasma density, and a mode jump in which the distribution of plasma density in the chamber changes occurs. When this phenomenon occurs, the light emission intensity of the plasma and the peak value of the bias voltage (Vpp voltage) change suddenly, and with this, the distribution of the etching rate in the wafer surface also changes largely, and therefore, power around the mode jump can not be used.
These three problems are also common to an ECR type plasma etching apparatus using pulsed discharge. But, in the above-described conventional techniques, these three problems are not considered.
Therefore, the present invention provides a plasma processing method and a plasma processing apparatus in which a stable process region can be ensured in a wide range, from low microwave power to high microwave power.